The antigens responsible for tumor rejection are very weak. The phenomenon of antigenic strength of tumors has been difficult to study because most comparisons have involved tumor lines which differed in many biologic characteristics. It is the purpose of this project to study antigenic strength of tumors in pairs of tumor sublines which differ in antigenic strength, but which are identical in other characteristics by virtue compared in a series of in vivo and in vitro tests for tumor immunity. The results will: 1) Provide information on how highly antigenic tumors differ from weakly antigenic tumors, perhaps leading to a classification of different types of "nonantigenic" tumors; 2) Provide evidence for or against relevance of the in vitro immunity assays to in vivo tumor resistance; 3) Provide examples of specific types of tumor nonrejection as targets for the development of new modes of immunotherapy.